Recently, a class of small non-coding RNAs, termed microRNAs (miRNAs), has been identified that function in post-transcriptional regulation of gene expression in plants and animals (Carrington and Ambrose, Science 301:336 (2003)). Originally identified in C. elegans, miRNAs act by basepairing to complementary sites in the 3′ untranslated region (UTR) or coding sequences of their target mRNAs and repressing their translation (Wang et al., Nucleic. Acids Res. 32:1688 (2004)).
While mature miRNAs are only ˜22 nucleotides (nt) in length, they originate from hairpin regions of ˜70 mer precursor (pre-miRNA) sequences through the action of Dicer complex (Lee et al., EMBO J. 21:4663 (2002)). The mature miRNA is then incorporated into the miRNP, the ribonucleoprotein complex that mediates miRNA's effects on gene regulation (Mourelatos et al., Genes Dev. 16:720 (2002)).
Bioinformatics studies predict that there are ˜100 miRNAs encoded in the worm and fly genomes, and ˜250 miRNAs encoded in the vertebrate genomes (Lai et al., Genome Biol. 4:R42 (2003); Lim et al., Genes Dev. 17:991 (2003); Lim et al., Science 299:1540 (2003)). This accounts for ˜0.5-1% of the number of predicted protein-coding genes for each genome, underlining the importance of miRNAs as a class of regulatory gene products (Brennecke and Cohen, Genome Biol. 4:228 (2003)).
miRNAs have been implicated in a variety of biological processes, including flower and leaf development in plants, larval development in worms, apoptosis and fat metabolism in flies, and hematopoietic differentiation and neuronal development in mammals (Bartel, Cell (2004)). In addition, many miRNA genes map to chromosomal regions in humans associated with cancer (e.g., fragile sites, breakpoints, regions of loss of heterozygosity, regions of amplification) (Calin et al., Proc. Natl. Acad. Sci. USA 101:2999 (2004)). Various miRNAs have also been shown to interact with the fragile X mental retardation protein (FMRP) in vivo (Jin et al., Nat. Neurosci. 7:113 (2004)), suggesting a role for these tiny RNAs in human health and disease.
Because different cell types and disease states are associated with expression of certain miRNAs, it is important to obtain both temporal and spatial expression profiles for miRNAs. Northern hybridization has been used to determine the expression levels of miRNAs (see, e.g., Sempere et al., Genome Biol. 5:R13 (2004); Aravin et al., Dev. Cell 5:337 (2003); Grad et al., Mol. Cell 11:1253 (2003); Lim et al., Genes & Dev. 17:991 (2003)), but this method is too labor intensive for high-throughput analyses. PCR-based methods have been used to monitor the expression of miRNAs, but these methods either require the use of costly gene-specific primers (see, e.g., Schmittgen et al., Nucleic Acids Res. 32:e43 (2004)) or inefficient blunt-end ligations to attach primer-binding linkers to the miRNA molecules (see, e.g., Miska et al., Genome Biol. 5:R68 (2004); Grad et al., Mol. Cell 11:1253 (2003); Lim et al., Genes & Dev. 17:991 (2003)). In addition, PCR can introduce significant biases into the population of amplified target miRNA molecules.
High-throughput microarrays have recently been developed to identify expression patterns for miRNAs in a variety of tissue and cell types (see, e.g., Babak et al., RNA 10:1813 (2004); Calin et al., Proc. Natl. Acad. Sci. USA 101:11755 (2004); Liu et al., Proc. Natl. Acad. Sci. USA 101:9740 (2004); Miska et al., Genome Biol. 5:R68 (2004); Sioud and Røsok, BioTechniques 37:574 (2004); Krichevsky et al., RNA 9:1274 (2003)). The use of microarrays has several advantages for detection of miRNA expression, including the ability to determine expression of multiple genes in the same sample at a single time point, a need for only small amounts of RNA, and the potential to simultaneously identify the expression of both precursor and mature miRNA molecules.
However, since mature miRNAs are only ˜22 nt in length and present in very limited quantities in any given tissue, these small RNAs present challenges for microarray labeling and detection (Sioud and Rosok, BioTechniques 37:574 (2004)). For example, covalent attachment of fluorophores can be used to directly label miRNA molecules for use in microarray analyses (see, e.g., Babak et al., RNA 10:1813 (2004); MICROMAX ASAP miRNA Chemical Labeling Kit, Perkin Elmer, Waltham, Mass.; Label IT® μArray Labeling Kit, Mirus Bio Corp., Madison, Wis.), but this method lacks the sensitivity to detect rare target miRNA molecules. Direct labeling can also result in intermolecular quenching of the randomly incorporated fluorophores, resulting in further decreased sensitivity. Random primed-reverse transcription of miRNA molecules has been used to produce labeled cDNA molecules for use in microarray analyses (see, e.g., Sioud and Rosok, BioTechniques 37:574 (2004); Liu et al., Proc. Natl. Acad. Sci. USA 101:9740 (2004)), but this method does not yield an accurate representation of the original full-length miRNA population.
New methods of labeling have been developed that have significantly improved both the accuracy and sensitivity of miRNA analysis (see, e.g., copending U.S. patent application Ser. No. 10/979,052, published as U.S. Patent Publication No. 2006/0094025). However, these methods utilize indirect label attachment and require multiple hybridization steps in order to develop the signal in the assay. Further, these methods are encumbered with large capture reagent molecules that require an independent hybridization step in order to improve the binding kinetics. While providing good results, these methods do not allow for easy adaptation to high through-put analysis and require significantly more time to achieve the desired results. As a result, there is an immediate need for rapid, sensitive and efficient methods for labeling and detection of miRNA molecules for use in microarray and high through-put analyses.